What is Scabies?
Scabies is a highly-contagious and itchy skin disease caused by tiny mites of the species Sarcoptes scabiei. The mites are transmitted by direct skin-to-skin contact; the more prolonged the contact, the greater the chance that scabies mites will be transferred from person to person. In most people, scabies infections do not consist of large numbers of Sarcoptes scabiei mites, and the rash that occurs is not due to the direct skin damage caused by the mites. Instead, the rash occurs due to an immune reaction to proteins on the mites themselves, the mites’ feces and the eggs of the scabies mites. The itchy scabies rash is basically an allergic reaction to the mites. The rash does not appear immediately after infection with mites, but several weeks after the initial infection, unless a person has contracted a scabies infection before. In this case, the rash occurs more quickly because the immune system is primed to respond to antigens on the S. scabiei mites. The scabies rash causes intense itching. The scabies rash may be confined to small areas of the body or more widespread, but it is not usually a severe health risk.
A small subset of scabies infections, however, progress to a more severe form, called Norwegian scabies or crusted scabies.
What is Norwegian Scabies?
Norwegian scabies is a much more severe form of the scabies infection that is usually only found in elderly individuals and people who have immune system deficiencies. For example, individuals with HIV/AIDS or genetic defects of the immune system are susceptible to a host of opportunistic diseases that people with healthy immune systems do not normally contract.
Just like a regular scabies infection, Norwegian scabies or crusted scabies is caused by the Sarcoptes scabiei mite. In people with Norwegian scabies, however, larger numbers of the mite are present. These individuals may harbor millions of these mites in their skin. This heavy mite infestation causes a rash that is widespread throughout the body, except for certain areas such as the skin of the face. Large areas of skin may become thick and crusted due to the mite infestation, hence the name “crusted scabies.” Norwegian scabies may not be as itchy as a regular scabies infection, but the thick skin may still have some level of itchiness. People with Norwegian scabies are highly contagious, even more so than those with regular scabies infections, because there are more mites that can be transferred to other people through direct skin-to-skin contact or contact with infested clothing or bedding.
Norwegian scabies is more difficult to treat than a regular scabies infection, not only due to larger numbers of the mites and mite eggs that may reinfect a person, but also because the thickened patches of skin are less permeable to scabicide medications that can kill scabies mites and their eggs. Thus, treatment for Norwegian scabies is generally prolonged and difficult.
Useful Resources
http://www.cdc.gov/parasites/scabies/disease.html
http://www.mayoclinic.com/health/scabies/DS00451
